The present invention relates to a high performance coated cutting tool insert particularly useful for turning of steel, like low alloyed steels, carbon steels and tough hardened steels under demanding conditions. The insert is based on WC, cubic carbides and a Co-binder phase with a cobalt enriched surface zone giving the cutting insert an excellent resistance to plastic deformation and a high toughness performance. Furthermore, the coating comprises a number of wear resistance layers which have been subjected to a surface post treatment giving the tool insert a surprisingly improved cutting performance.
The majority of today's cutting tools are based on a cemented carbide insert coated with several hard layers like TiC, TiCxNy, TiN, TiCxNyOz and Al2O3. The sequence and the thickness of the individual layers are carefully chosen to suit different cutting application areas and work-piece materials to be cut. The most frequently employed coating techniques are Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD). CVD-coated inserts in particular have a tremendous advantage in terms of flank and crater wear resistance over uncoated inserts.
The CVD technique is conducted at a rather high temperature range, from about 950 to about 1050° C. Due to this high deposition temperature and to a mismatch in the coefficients of thermal expansion between the deposited coating materials and the cemented carbide tool insert, CVD can lead to coatings with cooling cracks and high tensile stresses (sometimes up to 1000 MPa). Under some cutting conditions, the high tensile stresses can be a disadvantage as it may aid the cooling cracks to propagate further into the cemented carbide body and cause breakage of the cutting edge.
In the metal cutting industry there is a constant striving to increase the cutting condition envelope, i.e., the ability to withstand higher cutting speeds without sacrificing the ability to resist fracture or chipping during interrupted cutting at low speeds.
Important improvements in the application envelope have been achieved by combining inserts with a binder phase enriched surface zone and optimized thicker coatings.
However, with an increasing coating thickness, the positive effect on wear resistance is out balanced by an increasing negative effect in the form of an increased risk of coating delamination and reduced toughness making the cutting tool less reliable. This applies in particular to softer work piece materials such as low carbon steels and stainless steels and when the coating thickness exceeds from about 5 to about 10 μm. Further, thick coatings generally have a more uneven surface, a negative feature when cutting smearing materials like low carbon steels and stainless steel. A remedy can be to apply a post smoothing operation of the coating by brushing or by wet blasting as disclosed in several patents, e.g., EP 0 298 729, EP 1 306 150 and EP 0 736 615. In U.S. Pat. No. 5,861,210 the purpose has, e.g., been to achieve a smooth cutting edge and to expose the Al2O3 as the top layer on the rake face leaving the TiN on the clearance side to be used as a wear detection layer. A coating with high resistance to flaking is obtained.
Every post treatment technique that exposes a surface, e.g., a coating surface to a mechanical impact as, e.g., wet or dry blasting will have some influence on the surface finish and the stress state (σ) of the coating.
An intensive blasting impact may lower the tensile stresses in a CVD-coating, but often this will be at the expense of lost coating surface finish by the creation of ditches along the cooling cracks or can even lead to delamination of the coating.
A very intensive treatment may even lead to a big change in the stress state, e.g., from highly tensile to highly compressive as disclosed in U.S. Pat. No. 6,884,496, in which a dry blasting technique is used.
EP 1 734 155 discloses a CVD-coated cutting tool insert with a TiCxNy-layer with a low tensile stress level of from about 50 to about 390 MPa and an α-Al2O3-layer with a high surface smoothness with a mean Ra≦0.12 μm as measured by AFM-technique. This is obtained by subjecting the coating to an intensive wet blasting operation.